Field of the Invention
The present disclosure relates to a semiconductor device. Also, the present disclosure relates to a method to manufacture a semiconductor device.
Description of the Related Art
It is open to the public that a compound semiconductor device including a substrate, an electron transit layer formed over the substrate; an electron supply layer formed over the electron transit layer, and a buffer layer formed between the substrate and the electron transit layer and including AlXGa1-XN (0≤X≤1), wherein the x value represents a plurality of maximums and a plurality of minimums in the direction of the thickness of the buffer layer, and the variation of x in any area having a 1 nm thickness in the buffer layer is 0.5 or less (For reference, see unexamined Japanese patent publication No. JP2012-119586).
If a GaN-based high electron mobility transistor (HEMT) is used as a switching device of a power source, such a device is expected to reduce on-resistance and to have a higher breakdown voltage, however, there seem to be some problems: it is difficult to obtain crystalline GaN substrates in good quality, which tend to affect high-frequency characteristics of the transistor, also, the size of a semiconductor chip is required to be increased to obtain a higher breakdown voltage; furthermore, an electron-transit layer of a GaN-based HEMT is usually formed by use of metal-organic chemical vapor deposition (MOCVD) method, which requires a vacuum system that tends to increase cost, for example.
On the other hand, Gallium oxide (Ga2O3) exhibits wide band gap, which is wider than GaN, and attracts more attention as a potential semiconductor material for semiconductor devices. The band gap energy of Ga2O3 has been reported to be 4.8 to 53 eV. Also, Ga2O3 is known as a transparent semiconductor material, which hardly absorb visible light and/or ultraviolet light. Accordingly, Ga2O3 is considered to be a promising material for power devices, optical devices and electronic devices, some of which are operated and/or related to ultraviolet and/or deep ultraviolet region.
A light-emitting diode (LED) including an Sn-doped Ga2O3 film fabricated by MOCVD is open to the public (For reference, see NPL2 Jun Liang Zhao et al. “UV and Visible Electroluminescence From a Sn:Ga2O3/n+—Si Heterojunction by Metal-Organic Chemical Vapor Deposition”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 5 May 2011).
Also, Ga2O3 is known to possess five different polymorphs including α-, β-, γ-, δ-, and ε-phases (for reference, see NPL1: Rustum Roy et al, “Polymorphism of Ga2O3 and the System Ga2O3—H2O”). Among these five polymorphs, β-Ga2O3 is believed to be thermodynamically the most stable. Gallium oxide (Ga2O3) exhibits wide band gap and attracts more attention as a potential semiconductor material for semiconductor devices.
Furthermore, a Ga2O3 HEMT including an i-type β-Ga2O3 single crystal film, an n-type β-(AlxGa1-x)2O3 single crystal film formed on the i-type β-Ga2O3 single crystal film is open to the public (For reference, see international patent publication No, WO2013/035841). The Ga2O3 HEMT is composed of β-(AlxGa1-x)2O3 crystal (0<x≤0.6) containing a group IV element; a source electrode and a drain electrode, which are formed on the n-type β-(AlxGa1-x)2O3 single crystal film and a gate electrode formed on the n-type β-(AlxGa1-x)2O3 single crystal film and a gate electrode formed on the n-type β-(AlxGa1-x)2O3 single crystal film between the source electrode and the drain electrode.
β-Ga2O3 known to have β-Gallia structure, which is different from crystal structures generally used in electronic materials and devices, may not be always suitable for electronic materials and devices.
Compared with the number of researches of β-Ga2O3, the number of the researches of ε-Ga2O3 is still small, but it is suggested that a single crystal of ε-Ga2O3 is formed by HVPE (Halide Vapor Phase Epitaxy) method (For reference, see unexamined Japanese patent publication No. 2017-07871, and NPL3: Yuichi OSHIMA, et al., “Epitaxial growth of phase-pure ε-Ga2O3 by halide vapor phase epitaxy”, Journal of applied physics 118, 085301, 2015). Also, it is suggested that ε-Ga2O3 thin films are formed by mist Chemical Vapor Deposition (CVD) (see NPL4: Hiroyuki NISHINAKA, et al. “Heteroepitaxial growth of ε-Ga2O3 thin films on cubic (111) MgO and (111) yttria-stabilized zirconia substrates by mist chemical vapor deposition”, published online Nov. 11, 2016).